U.S. patent application number 11/605912 was filed with the patent office on 2008-05-29 for electro-magnetic acoustic measurements combined with acoustic wave analysis.
This patent application is currently assigned to BAKER HUGHES INCORPORATED. Invention is credited to Vitaly N. Dorovsky, Vladimir Dubinsky, Xiao Ming Tang.
Application Number | 20080125974 11/605912 |
Document ID | / |
Family ID | 39464727 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080125974 |
Kind Code |
A1 |
Dubinsky; Vladimir ; et
al. |
May 29, 2008 |
Electro-magnetic acoustic measurements combined with acoustic wave
analysis
Abstract
A method and apparatus for assessing the permeability of a
subterranean formation and the hydrocarbon and/or water content of
the formation. The method includes emitting an acoustic signal,
such as a Stoneley wave into the formation and sending an
electro-magnetic pulse into the formation. An analysis of the
response of the Stoneley wave in conjunction with an analysis of a
measurement of the electrical potential within the wellbore
provides information pertinent to permeability and fluid
composition.
Inventors: |
Dubinsky; Vladimir;
(Houston, TX) ; Dorovsky; Vitaly N.;
(Novosibirsky, RU) ; Tang; Xiao Ming; (Sugar Land,
TX) |
Correspondence
Address: |
KEITH R. DERRINGTON;BRACEWELL & GUILIANI LLP
P.O. BOX 61389
Houston
TX
77002-2781
US
|
Assignee: |
BAKER HUGHES INCORPORATED
|
Family ID: |
39464727 |
Appl. No.: |
11/605912 |
Filed: |
November 29, 2006 |
Current U.S.
Class: |
702/11 |
Current CPC
Class: |
Y02A 90/344 20180101;
Y02A 90/30 20180101; G01V 3/265 20130101 |
Class at
Publication: |
702/11 |
International
Class: |
G01V 11/00 20060101
G01V011/00 |
Claims
1. A method of evaluating a characteristic of a subterranean
formation comprising: transmitting an acoustic signal into the
formation; evaluating the response of the acoustic signal; emitting
an electro-magnetic signal into the formation; measuring the
response of the electro-magnetic signal; and assessing the fluid
composition of the formation based on the measured response.
2. The method of claim 1 wherein the acoustic signal comprises a
Stoneley wave.
3. The method of claim 1 wherein the acoustic signal is selected
from a list consisting of a Raleigh wave, a compressional wave, a
shear wave, and combinations thereof.
4. The method of claim 1, wherein analysis of the acoustic signal
response provides information relating to the permeability of the
formation.
5. The method of claim 1 wherein the step of assessing the fluid
composition comprises determining the amount of water residing in
the formation.
6. The method of claim 1 wherein the step of assessing the fluid
composition comprises determining the amount of hydrocarbon
residing in the formation.
7. The method of claim 1 wherein the acoustic signal motivates
fluid within the formation, wherein the fluid motivated contains
electrolytes.
8. The method of claim 6 wherein movement of the electrolytically
containing fluid creates an electrical current within the
formation.
9. The method of claim 7 further comprising measuring the
electrical current.
10. The method of claim 8 further comprising estimating the water
content of the formation based on the measure of the electrical
current.
11. The method of claim 8 further comprising estimating the
hydrocarbon content of the formation based on the measure of the
electrical current.
12. A downhole tool comprising: an acoustic transmitter; an
acoustic receiver; a voltage potential measurement device; and an
electro-magnetic source, wherein the voltage potential measurement
device can measure the axial potential difference along the tool
due to electro-acoustic effects of a permeable media.
13. The downhole tool of claim 12, wherein the acoustic transmitter
emits Stoneley waves.
14. The downhole tool of claim 12, wherein transmitting the
acoustic transmitter into media surrounding the tool creates an
electro-acoustic current.
15. The downhole tool of claim 14, wherein the electro-magnetic
source creates a magnetic field tangential to the direction of the
electro-acoustic current.
16. The downhole tool of claim 12 wherein the voltage potential
measurement device comprises electrodes on the downhole tool.
17. The downhole tool of claim 16, wherein the electrodes comprise
rings coaxially disposed on the outer surface of the downhole
tool.
18. The downhole tool of claim 12 wherein the electro-magnetic
source is a magnet.
19. A method of evaluating a borehole comprising: emitting a
Stoneley wave within the borehole; monitoring the emitted Stoneley
wave; emitting an electro-magnetic signal within the borehole;
measuring electrical potential along the wellbore; and assessing a
characteristic of the formation adjacent the wellbore based on
monitoring the Stoneley wave and the measurement of electrical
potential.
20. The method of claim 19, wherein a characteristic of the
formation comprises the fluid composition of the formation.
21. The method of claim 19 further comprising emitting an acoustic
signal within the wellbore, wherein the acoustic signal is selected
from a list consisting of a Raleigh wave, a compressional wave, a
shear wave, and combinations thereof.
22. The method of claim 19 wherein the step of assessing the fluid
composition comprises determining the amount of water residing in
the formation.
23. The method of claim 20 wherein the fluid composition comprises
the amount of hydrocarbon residing in the formation.
24. The method of claim 19 wherein the acoustic signal motivates
fluid within the formation, wherein the fluid motivated contains
electrolytes.
25. The method of claim 24 wherein movement of the electrolytically
containing fluid creates an electrical current within the
formation.
26. The method of claim 25 further comprising estimating the water
content of the formation based on the measure of the electrical
current.
27. The method of claim 25 further comprising estimating the
hydrocarbon content of the formation based on the measure of the
electrical current.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure relates to a device and a method for
evaluating the characteristics of a subterranean formation. More
specifically, the present disclosure relates to a device and method
for evaluating the permeability of a hydrocarbon producing
wellbore. Yet more specifically, the present disclosure concerns a
device and method employing both acoustic and electro-magnetic
transmission and receiving means for the evaluation of a
subterranean formation.
[0003] 2. Description of Related Art
[0004] Knowing certain characteristics of the subterranean
formation surrounding a borehole, such as permeability, porosity,
rugosity, skin factors, and other such properties can be used to
estimate hydrocarbon-producing capability from these formations.
Some of the more fundamental reservoir properties include
permeability and relative permeability, along with porosity, fluid
saturations, and formation pressure. Knowing these properties is
also useful in evaluating the presence of water along with the
presence of hydrocarbons.
[0005] With regard to subterranean formations, permeability is a
measure of the ability to flow fluids through the material making
up the formation, the material is typically rock or unconsolidated
alluvial material. Permeability is generally measured in units of
darcy, milli-darcy or md (1 darcy.apprxeq.10.sup.-12 m.sup.2).
Permeability represents the relationship between flow through a
medium and physical properties of that fluid to a pressure
differential experienced by the fluid when flowing through the
medium. For a subterranean formation to produce liquid hydrocarbon,
it should have a permeability of at least 100 md, to produce gas
hydrocarbon the permeability can be lower.
[0006] A determination of formation permeability can be obtained by
taking a core sample. However coring techniques have some
drawbacks, such as time, expense, and inaccuracies due to sample
errors and limited sample amounts. Other techniques for evaluating
permeability include formation testing tools that actually
penetrate the wellbore wall and draw connate formation fluid into
the tool. The actual fluid as well as the amount of fluid flowing
into the tool can be evaluated in order to make permeability
determinations. Formation testing tools however are subject to
inherent errors, such as pressure differentials between the
formation and the tool that allow portions of the connate fluid to
vaporize, thereby altering the fluid composition. When the original
connate fluid is allowed to change phase, a determination of
porosity is made more difficult. Additionally, since the tool must
pierce the borehole wall, mudcake present in that wall can lodge
itself in the probe tip thereby precluding the taking of a
representative sample of connate fluid.
BRIEF SUMMARY OF THE INVENTION
[0007] The present disclosure includes a method of evaluating a
characteristic of a subterranean formation, where the formation
characteristic includes permeability as well as hydrocarbon and/or
water content. The method includes transmitting an acoustic signal
into the formation, evaluating the response of the acoustic signal,
emitting an electro-magnetic signal into the formation, measuring
the response of the electro-magnetic signal, and assessing the
fluid composition of the formation based on the measured response.
The acoustic signal can comprise a Stoneley wave. The acoustic
signal can also be a Raleigh wave, a compressional wave, a shear
wave, and combinations thereof. The analysis of the acoustic signal
response provides information relating to the permeability of the
formation. The step of assessing the fluid composition comprises
determining the amount of water residing in the formation and
determining the amount of hydrocarbon residing in the
formation.
[0008] Also included within the scope of this disclosure is a
downhole tool, the tool may comprise an acoustic transmitter, an
acoustic receiver, a voltage potential measurement device, and an
electro-magnetic source. The voltage potential measurement device
of this tool can measure the axial potential difference along the
tool due to electro-acoustic effects of a permeable media. The the
acoustic transmitter may emit Stoneley waves. The electro-magnetic
source may create a magnetic field tangential to the direction of
the electro-acoustic current. The voltage potential measurement
device may comprise electrodes on the downhole tool, the electrodes
may involve rings coaxially disposed on the outer surface of the
downhole tool. Optionally, the electro-magnetic source may be a
magnet.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0009] FIG. 1 illustrates an embodiment of a downhole tool disposed
within a wellbore.
[0010] FIG. 2 shows a downhole tool along with examples of measured
responses of the tool.
DETAILED DESCRIPTION OF THE INVENTION
[0011] With reference now to FIG. 1, one example of a downhole tool
10 in accordance with the present disclosure is shown. In the
embodiment of FIG. 1, the tool 10 comprises a transmitter 12, a
receiver array 14, electrodes 18, and an electro-magnetic source
22. The transmitter 12 should be capable of producing acoustic
waves, wherein the waves are transmissible into the formation
surrounding the borehole 2. The acoustic waves generated by the
transmitter 12 include Stoneley waves, compression waves (P type),
shear waves (including shear horizontal SH and shear vertical SV).
The receiver array 14 should be configured to receive the
corresponding the wave signal. As shown, the transmitter 12 is a
single point source disposed above the array 14, and the receiver
array 14 comprises a series of individual receivers 16. However the
transmitter 12 may comprise more than one source (i.e. multiple
transmitters) and the receiver array 14 could be comprised of a
single receiver 16. Additionally, the transmitter 12 can be
disposed beneath the receiver array 14, or an additional array may
be included wherein the transmitter 12 resides between the arrays.
Moreover, different phasing arrangements of the receiver array 14
are possible, where the individual receivers 16 are phased at
varying locations around the circumference of the tool 10.
[0012] The electro-magnetic source 22 can comprise a conductor
capable of producing a large pulse current along the axis of the
tool. The pulse current can create a strong magnetic field in a
direction tangential to the axis of the tool and, as will be
described in more detail below, perpendicular to electro-acoustic
current flowing within the formation. Alternatively, the
electro-magnetic source 22 can comprise a permanent magnet. The
electro-magnetic source 22 as shown in FIG. 1 is disposed within
the body of the downhole tool 10, however other arrangements are
possible as well, such as on the outer surface of the tool 10.
Although depicted between the transmitter and receivers, the
electro-magnetic source 22 on either side of the electrodes as well
as the transmitters or receivers.
[0013] Formation permeability and the possible presence of
hydrocarbon and/or water in a reservoir can be determined by
analyzing a formation with a combination of an acoustic signal and
an electro-magnetic impulse. In situations when the formation
contains a conductive fluid (i.e. a fluid containing an
electrolyte), applying an acoustic signal to a rock matrix within
the formation can produce relative motion of the conductive fluid
within the rock matrix. A Stoneley wave is one such example of an
appropriate acoustic signal. This relative motion, in turn, creates
an electrical current due to electro-chemical effects taking place
at the surface of the boundary between the matrix and electrolyte.
A seismic signal or wave traveling through the fluid saturated rock
matrix creates a resulting electrical field because the pore fluid
within the matrix carries an excess electrical charge. The excess
electrical charge is stored within the fluid because of ions
absorbed by minerals in the matrix have a particular polarity. The
absorption of these polarized ions in the matrix results in ions of
an opposite polarity having a dominant presence in the fluid,
thereby resulting in a charged solution. Thus if a resulting
pressure gradient, such as that produced by a seismic signal, urges
the solution through the rock matrix the fluid movement along the
pore surface moves the particularly charged ions thereby creating a
streaming electrical current. The streaming electrical current in
turn induces an electrical field in the formation.
[0014] Stoneley (sometimes call "tube") waves are high-amplitude
guided waves generated by a radial flexing of the borehole as the
acoustic energy passes from the borehole fluid into the rock
formation. They propagate at low frequencies along the fluid/rock
interface at the borehole wall. Stoneley waves are notable for
several special properties: there is no cut-off frequency;
dispersion is very mild; for all frequencies, Stoneley-wave
velocity is less than fluid velocity; group velocity nearly equals
phase velocity over the frequency range. The Stoneley wave has
maximum amplitude at the wall of the borehole, and decays radially
away from the wall. At low frequencies Stoneley-wave velocity is
calculated as follows:
VST=[.rho.(1/K+1/G)].sup.1/2
[0015] where .rho., K, and G are defined as: [0016] K=bulk modulus
of elasticity [0017] G=shear modulus of elasticity [0018]
.rho.=density of medium The energy/amplitude of acoustic waves is
attenuated or dispersed primarily by travel through the borehole
fluid and rock matrix. Additional attenuation is usually caused by
the following factors: particle friction, changes in acoustic
impedance (the product of density (.rho.) and acoustic velocity
(v)) at interfaces between different mediums, borehole rugosity,
mudcake rigidity and signal reduction due to tool eccentricity. The
radial flexing of the borehole caused by the Stoneley wave creates
an extended zone of the borehole just adjacent a compressed region.
Fluid within the rock matrix flows from the formation adjacent the
compressed region to the extended region. A net charge differential
is then created between the compressed and extended regions. The
actual charge (i.e. positive or negative) of each region is
dependent on the charge of the pore fluid. For example, if the pore
fluid has a positive charge, then the region proximate to the
extended region will have a generally positive charge due to fluid
migration caused by the seismic signal. An indication of a
permeable formation exists when the depth dependence of travel time
delay corresponds to the centroid frequency shift.
[0019] In one example of use of the device of FIG. 1, the downhole
tool 10 is disposed within a wellbore 5 for investigating the
wellbore 2 and the formations adjacent the wellbore 2. One example
of such an investigation comprises activating the transmitter 12
thereby creating an acoustic signal that is then transmitted into
the formation adjacent the wellbore 2. The resulting signal,
wherein at least a portion of which has passed through the
formation, can then be received and recorded by the receiver array
14. While disposed in the wellbore 2, the electro-magnetic source
22 is activated to project a magnetic field 20 into the formation
surrounding the borehole 2. Alternatively, a strong permanent
magnet can be used. Preferably the magnetic field 20 is directed
substantially perpendicular to the flow of the streaming current.
However other orientations of the magnetic field 20 can yield
useful results. It is believed that those skilled in the art can
ascertain these other orientations without undue experimentation.
During the acoustic sampling of the borehole 2, an investigation of
electrical potential can be conducted as well. One example of
electrical sampling using the device of FIG. 1 involves measuring
the electrical potential that exists between the electrodes 18.
While the electrodes 18 of FIG. 1 are shown as ring electrodes,
they can take on any form capable of measuring an electrical
potential along the downhole tool 10, either axially or radially.
Also, when using the tool 10, readings can occur continuously while
lowering or raising the tool 10, or at discrete locations within
the wellbore 2. The pulse current source can be initiated as soon
as the tool 10 is disposed in the wellbore 2, or when desired to
take measurements in the wellbore 2.
[0020] FIG. 2 demonstrates a situation where the acoustic signal
emitted by the transmitter 12 is a Stoneley wave. In this figure an
example borehole 2a is provided along with example subterranean
formations adjacent to the borehole 2a, e.g. non-permeable
formation 4, permeable formation 6, and mud cake 8. FIG. 2 also
includes examples of measured responses aligned to correspond with
the different formations of the borehole 2a. Thus one of the
advantages of employing embodiments of the device and method herein
disclosed is to identify types of formations within the borehole 2a
by analyzing the measured responses individually and in
combination. The measured responses include: (1) a measured
velocity of a Stoneley wave in the example borehole 2a, and (2)
electrical potential measured by the electrodes 18 in the example
borehole 2a. Based on a study of these responses, a potential
hydrocarbon producing zone can be identified by using the tool of
FIG. 1. For example, as shown in FIG. 2, a decrease in the measured
velocity of the Stoneley wave is shown corresponding to the region
of the permeable formation 6 encountered by the downhole tool 10.
The measured response of electro-acoustic voltage similarly
responds to the presence of the permeable zone 6 with an increase
for water saturated and a stable value thereby indicating a
hydrocarbon presence.
[0021] The electrical current effect will be measurably conspicuous
in the case of a (conductive) water-saturated formation, conversely
it will be substantially inconspicuous if the formation is 100%
oil-saturated. Partial presence of an electrolyte (conductive
water) in the permeable media will reduce this effect due to
blocking effects of the oil on a portion of the boundary surface
between the electrolyte and the rock matrix. As discussed below, a
measurement of the electrical current produced in the formation by
the relative motion, can indicate hydrocarbon/water ratios present
in the formation. Measuring the electro-acoustic current in this
case could allow an indicative estimation of the partial oil
content in this mixture.
[0022] Conducting these measurements while drilling could
significantly reduce negative effects of the mud cake/invaded zone
on the interpretation of these measurements. Presence of these
additional factors can be taken into account by including a model
of the mud cake/invasion zone formation into the electro-acoustic
model.
[0023] The present invention described herein, therefore, is well
adapted to carry out the objects and attain the ends and advantages
mentioned, as well as others inherent therein. While a presently
preferred embodiment of the invention has been given for purposes
of disclosure, numerous changes exist in the details of procedures
for accomplishing the desired results. For example, the device is
not limited to being wireline conveyed, but can be suspended by any
known means, such as tubing, coiled tubing, or slickline as well as
any later developed means. Additionally, the device and method can
be used in conjunction with downhole drilling or other boring
operations. These and other similar modifications will readily
suggest themselves to those skilled in the art, and are intended to
be encompassed within the spirit of the present invention disclosed
herein and the scope of the appended claims.
* * * * *